Reinforcing ribs for tanks have been made on a commercial scale by winding resin-impregnated glass filaments over hollow rib forms secured to the exterior of a tank wall until an adequate rib thickness has been built up. U.S. Pat. No. 3,818,950 teaches that the filament windings may be applied circumferentially, with no helix angle. U.S. Pat. Nos. 3,661,294 and 3,700,512 teach that the filament windings can be cross wound around a rib form to produce a rib. U.S. Pat. No. 3,412,891 teaches several approaches to the formation of an external rib on a reinforced plastic storage tank. Each patent teaches the use of heat-curable resin to impregnate glass filaments just before they are wrapped around a hollow rib form. After an adequate rib thickness has been built up, the wrapping is stopped and heat is applied to cure the resin.
The use of glass filaments, impregnated with a heat-curable resin, to a rib structure is very time consuming and, hence, expensive. A very large number of wrappings of glass filaments is required to produce a rib with adequate thickness and strength. Once the wrapping process is completed, there is a substantial period of time required to cure the lay-up. In the case of a double wall tank, the fabrication of an outer wall over the ribs must wait until the ribs have cured completely. U.S. Pat. No. 4,676,093 discloses a double wall tank with annular ribs bonded to and connecting the inner and outer walls.